feat: automatic sync upstream using Github Actions

After forking the project, due to the limitations imposed by GitHub, you need to manually enable Workflows and Upstream Sync Action on the Actions page of the forked project.

.github/workflows/sync.yml

@@ -0,0 +1,40 @@
+name: Upstream Sync
+
+permissions:
+  contents: write
+
+on:
+  schedule:
+    - cron: "0 0 * * *" # every day
+  workflow_dispatch:
+
+jobs:
+  sync_latest_from_upstream:
+    name: Sync latest commits from upstream repo
+    runs-on: ubuntu-latest
+    if: ${{ github.event.repository.fork }}
+
+    steps:
+      # Step 1: run a standard checkout action
+      - name: Checkout target repo
+        uses: actions/checkout@v3
+
+      # Step 2: run the sync action
+      - name: Sync upstream changes
+        id: sync
+        uses: aormsby/Fork-Sync-With-Upstream-action@v3.4
+        with:
+          upstream_sync_repo: ccbikai/BroadcastChannel
+          upstream_sync_branch: main
+          target_sync_branch: main
+          target_repo_token: ${{ secrets.GITHUB_TOKEN }} # automatically generated, no need to set
+
+          # Set test_mode true to run tests instead of the true action!!
+          test_mode: false
+
+      - name: Sync check
+        if: failure()
+        run: |
+          echo "[Error] 由于上游仓库的 workflow 文件变更，导致 GitHub 自动暂停了本次自动更新，你需要手动 Sync Fork 一次。"
+          echo "[Error] Due to a change in the workflow file of the upstream repository, GitHub has automatically suspended the scheduled automatic update. You need to manually sync your fork."
+          exit 1